Bidisperse Magnetorheological Fluids using Fe Particles at Nanometer and Micron Scale

TLDR

Conventional MR fluids consist of micron‑sized particles in a carrier fluid, and recent research explores bidisperse mixtures of two particle sizes to improve performance. The study seeks to identify the optimal composition of bidisperse Fe particles that balances high yield stress with low settling rate by examining how increasing the weight percent of 30 nm particles relative to 30 mm particles affects rheological properties. The authors prepared 60 wt% Fe bidisperse MR fluids comprising micron‑ and nanometer‑sized particles, measured their flow curves, applied a Bingham‑plastic model, and found that adding nanoparticles reduces settling by filling interstitial pores while altering rheology. Increasing the weight percent of nanometer‑sized Fe particles lowers settling rate but also reduces maximum yield stress, and rheological properties vary predictably with the nanometer‑to‑micron particle ratio at constant solids loading.

Abstract

Conventional magnetorheological (MR) fluids are suspensions of micron-sized particles in a hydraulic or silicone oil carrier fluid. Recently, research has been conducted on the advantages of using bidisperse fluids, which are mixtures of two different powder sizes in the MR suspension. The MR fluids investigated here use a mixture of conventional micron- sized particles and nanometer-sized particles. The settling rate of such bidisperse fluids using nanometer-sized particles is reduced because the nanoparticles fill pores created between the larger particles, thereby reducing fluid transport during creeping flow. This reduction in the settling rate comes at a cost of a reduction in the maximum yield stress that can be manifested by such an MR fluid at its saturation magnetization. There is a measurable and predictable variation in rheological properties as the weight percent (wt%) of the nanometer-sized particles is increased relative to the weight percent (wt%) of micron-sized particles, while maintaining a constant solids loading in the MR fluid samples. All bidisperse fluids tested in this study have a solids loading of 60 wt% of iron (Fe) particles. This study investigates the effect of increasing the wt% of 30 nm (nominal) Fe particles relative to 30 mm (nominal) Fe particles on rheological characteristics, such as yield stress and postyield viscosity. The goal of this study is to find an optimal composition of the bidisperse fluid that provides the best combination of high yield stress and low settling rate based on empirical measurements. The applicability of the Bingham-plastic rheological model to the measured flow curves of these MR fluids is also presented.